The present invention relates to a conductive coil contact member suitable for use in contact probes and contact cards for testing semiconductor devices and wafers, electric sockets for devices in such forms as LGA (land grid array), BGA (ball grid array), CSP (chip side package) and bare chip, and electric connectors. The present invention also relates to a contact probe head using a conductive coil contact member.
Conventionally, various forms of conductive contact members have been used in electrically testing (open/short circuit tests, environmental tests, burn-in tests and so forth) electroconductive patterns of printed circuit boards and electronic components, and have been widely used in contact probes for testing wafers, electric sockets for semiconductor devices in such forms as LGA, BGA, CSP and bare chip, and connectors.
In applications to electric sockets for semiconductor devices, for instance, there is a trend for using higher signal frequencies for semiconductor devices, and frequencies in the order of hundreds of MHz have become very common. In a socket for a semiconductor device which operates under such high frequency signals, the conductive contact member which forms a part of the electrically conductive path for the device is required to have an even lower inductance and resistance. For instance, U.S. Pat. No. 6,043,666 issued Mar. 26, 2000 to T. Kazama discloses a coil spring having a closely wound conical end which serves as a contact member so that the contact member and compression coil spring are integrally combined, and the inductance and resistance are reduced.
In addition to the need to reduce the electric inductance and resistance, it is desired to ensure the positional accuracy of each contact member with respect to the object to be contacted particularly when a large number of such conductive contact members are arranged in a support member to access a large number of points at the same time.
A conductive coil contact member allows the electric inductance and resistance to be minimized, but causes some difficulty in achieving the positional accuracy of its tip with respect to the object to be contacted. The tip is formed by conically winding a coil wire, but the last turn of the coil wire has a certain radius, and the position of the tip of the cone or the highest point of the cone could be anywhere along the last turn which is offset from the axial center of the cone by this radius. In short, the position of the tip of a conically wound contact member is offset from the axial center of the cone by a distance equal to the radius of the last turn of the coil wire.
Such an offset can be reduced by reducing the radius of the last turn, but it is difficult to reduce the radius of a turn beyond a certain limit for a given diameter of the coil wire. If a coil wire having a small diameter is used, it is possible to reduce the radius of the turn, but the desired resiliency or rigidity of the contact member may not be achieved, and the electric inductance and resistance inevitably increase.
U.S. Pat. No. 6,043,666 discloses a conductive coil contact member having each coil wire end formed into a pin extending centrally in the axial direction. Such an arrangement permits the positional accuracy of each tip to be increased without any limit in theory, but becomes impractical when the size of the conductive coil contact member is reduced beyond a certain level.
In view of such problems of the prior art, a primary object of the present invention is to provide a conductive coil contact member which demonstrates a low electric inductance and resistance while providing a high level of positional accuracy for the contact point thereof.
A second object of the present invention is to provide a conductive coil contact member which provides a high level of positional accuracy for the contact point thereof but is easy and economical to manufacture.
A third object of the present invention is to provide a conductive coil contact member which demonstrates both mechanically and electrically favorable properties.
A fourth object of the present invention is to provide an improved contact probe using a conductive coil contact member.
According to the present invention, such objects can be accomplished by providing a conductive coil contact member having at least one tapered end consisting of a plurality of turns of coil wire having a progressively smaller coil radius toward a free end thereof, wherein: the coil wire comprises a core wire and at least one highly electrically conductive layer formed over the core wire, a last turn of the coil wire at the free end having a smaller coil radius than would be possible by coiling the coil wire.
Thus, the core wire is coiled to a smallest possible radius in the last turn, and the coil wire diameter is thereafter increased by forming layers formed by plating or other similar methods. The final result is that the last turn of the coil wire at the free end has a smaller coil radius than would be possible by coiling the coil wire. The reduction in the coil radius of the last turn contributes to the improvement in the positional accuracy of the free end of the tapered end of the conductive coil contact member.
According to a preferred embodiment of the present invention, a plurality of layers are formed over the core wire, the layers including at least one highly electrically conductive layer, made of such material as silver, copper and alloy of such a material, and at least one layer having a favorable mechanical property such as nickel. The outer layer may consist of material which is resistant to corrosion and oxidization such as gold, rhodium and alloy of such material. The core wire may be made of material having a favorable spring property such as steel.
The electrically conductive layer may either simply cover the coil wire or may continuously extend between adjacent turns of the coil wire so that a favorable electric path extending in the axial direction may be achieved instead of a spiral path.
The present invention also provides a contact probe head, comprising: a holder consisting of at least one plate member and having at least one holder hole extending across a thickness of the plate member; and a conductive coil contact member received in the holder hole, the coil contact member having a first tapered coil end consisting of a plurality of turns of coil wire having a progressively smaller coil radius toward a free end thereof, an intermediate coil spring potion including a coarsely wound section, and a second coil end which also consists of a tapered coil end consisting of a plurality of turns of coil wire having a progressively smaller coil radius toward a free end thereof, the free end of the second tapered coil end being soldered to a terminal of an external base board.
The soldering connection for the second tapered end ensures both the positional stability and favorable electric property of the conductive coil contact member. Because of the conical shape of the second tapered end, it is possible to ensure a stable connection even if the amount of applied solder is small owing to the small cross sectional area of the free end of the second tapered end, and prevent the solder from reaching the compression coil spring portion and impairing the spring function thereof owing to the capability of the second tapered end to retain a pool of solder inside the tapered turns of the coil wire.
Similar results can be achieved if the second coil end consists of a plurality of turns of coil wire wound radially inwardly substantially in a plane perpendicular to an axial line of the coil contact member so as to define a substantially flat end surface, instead of a tapered end.
In such a contact probe head, for the ease of soldering work and other assembly processes, it is desirable to be able to retain each conductive coil contact member in the corresponding holder hole while allowing the conductive coil contact member to be installed in the holder hole without any substantial difficulty. To achieve this goal, according to a certain aspect of the present invention, the contact probe head of the present invention may comprise a holder consisting of at least one plate member and having at least one holder hole extending across a thickness of the plate member; a conductive coil contact member received in the holder hole; and means for resiliently retaining the contact member in the holder hole.
The retaining means may comprise a section of the conductive coil contact member having a slightly larger coil outer diameter than a corresponding inner diameter of the holder hole, or a plurality of radial projections provided in a part of the holder hole such as an open end thereof, a circle defining by inscribing free ends of the radial projections being slightly smaller than a corresponding coil outer diameter of the conductive coil contact member. Alternatively, the retaining means may comprise a plurality of radial projections extending over a substantial length of the holder hole on an inner circumferential surface of the holder hole, and a section of the conductive coil contact member having a slightly larger coil outer diameter than a circle defining by inscribing free ends of the radial projections.